Process for non-contact distance measurement

ABSTRACT

In a procedure for non-contact distance measurement, in particular to measure the distance from obstacles in the immediate vicinity of motor vehicles, measurements are made using a first measuring procedure, and in a first measuring range absolute values of the respective distance are determined. Using a second measuring procedure, relative values are determined in a second measuring range that overlaps the first. Absolute values of the first measuring procedure, which lie inside the overlapping range, are used to calibrate the relative values of the second measuring procedure.

FIELD OF THE INVENTION

The present invention relates to a procedure for non-contact distancemeasurement, in particular for measuring the distance from obstacles inthe immediate vicinity of motor vehicles.

BACKGROUND INFORMATION

Various distance measuring procedures are known for collision warningdevices also known as parking aids. German Patent Application No. 40 23538 describes a collision warning device based on ultrasound in which atleast two ultrasound sensors are mounted at a specified distance fromeach other. In this procedure, the delay times between the time when anultrasound signal is emitted and a reflected ultrasound signal isreceived from the same and from the other ultrasound sensors aremeasured and evaluated.

The measuring range of ultrasound sensors is limited by the beam angle,the size of the obstacle, its reflective properties and by the detectionthreshold of the sensor. This means, for one thing, that a "dead angle"results for each of the ultrasound sensors. In addition, within therecording angle, no distance measurement will be carried out within theimmediate vicinity because of the decay period of the sensors.

In contrast, capacitive sensors have the advantage that measurement ispossible up to the shortest distances. In addition, if the sensor isdesigned appropriately, a measurement without voids can be carried outover the entire vehicle width. However, these advantages are counteredby the disadvantage that, due to the various materials, sizes and shapesof the possible obstacles, no firm connection exists between thedistance and the measurable capacitance in each case, i.e. only relativedistance values are determined.

British Patent No. 2,266,397 describes a motor vehicle parking aid inwhich a distance to a possible obstacle is monitored using an ultrasoundsensor. Since the ultrasound sensor cannot monitor all areas, inparticular not the area in the proximity of the vehicle, a secondmeasuring system is provided. This measuring system is configured as acapacitive system and detects obstacle at a minimum distance from thevehicle. The publication, Kramar E., "Funksysteme fur Ortung undNavigation" (Radio systems for location and navigation), 1973, Verl.Berliner Union GmbH, Stuttgart, Germany, pp. 28 and 29, discloses aphase difference measuring process for measuring the distance betweentwo radiation sources A and B, in which radiation sources A and Btransmit in-phase HF signals, received at the receiving station with acertain phase difference.

The object of the present invention is to provide a procedure fornon-contact distance measurement, in particular for measurement of thedistances from obstacles in the immediate vicinity of motor vehicles, inwhich measurement of short distance values is also possible.

SUMMARY OF THE INVENTION

This object is achieved by using the present invention in thatmeasurements are made using a first measuring procedure in whichabsolute values are determined for the current distances in a firstmeasuring range; that using a second measuring procedure relative valuesare determined in the measuring range that overlaps the first one, andthat absolute values of the first measuring procedure which lie withinthe overlapping area are used to calibrate the relative values from thesecond measuring procedure.

The procedure according to the present invention has the advantage thatthe distance measurement calibration is performed using the secondmeasuring procedure, which does not give any absolute values per se, sothat ultimately absolute values will be available.

Although the process for the procedure according to the presentinvention can be implemented using a number of measuring procedures, itis advantageous if the first measuring procedure is based on a delaytime measurement of waves emitted, reflected and received again and thesecond measuring procedure is based on a measurement of an electrical,magnetic, or optical variable. According to the present invention, thefirst measuring procedure will be preferably carried out usingultrasound waves and the second measuring procedure measures acapacitance that is formed between an electrode and the environment withthe obstacles.

This further development has the advantage that, with an appropriateelectrode that extends over the entire width of the vehicle, ameasurement of the distance without voids is possible over the entirewidth of the vehicle.

One of the advantageous embodiments of the procedure according to thepresent invention consists of storing a relationship between therelative values determined using the second measuring procedure and theactual distance for various parameters in a map consisting of a numberof characteristic curves and selecting one characteristic curve forcalibration, which will be used in the following measurements using thesecond measuring procedure by inputting at least one absolute valuedetermined using the first measuring procedure as the actual distanceand the simultaneously determined relative value. To do this, theindividual characteristic curves can be recorded by empirical tests withvarious obstacles.

Another advantageous embodiment according to the present inventionconsists of storing a relationship between the relative valuesdetermined using the second measuring procedure, the actual distance,and a calibration factor as an equation and calculating the calibrationfactor for calibration by inputting at least one absolute valuedetermined using the first measuring procedure as the actual distanceand the simultaneously determined relative value which in the followingmeasurements is used using the second measuring procedure. In thissimplified embodiment, a similar behavior is assumed for variousobstacles.

The absolute values provided for calibration using the first measuringprocedure can be subject to an interference from various influences.Also the interference signals can be superimposed on the output signalof the sensor used for the second measuring procedure. Therefore,according to a further development of the procedure according to thepresent invention, several absolute values lying in the overlapping areaare used for calibration. In this process, greatly deviating absolutevalues can be disregarded.

The frequent uses of the procedure according to the present inventionresult during approach to an obstacle. In this process, at first,measuring results from the first measuring procedure are available. Ifthen an overlapping measuring range is reached, considering the lateralposition of the obstacle, due to the distance diminishing, thecalibration for the second measuring procedure follows. With furtherdecreasing distance, the first measuring procedure then supplies no moremeasurements; these are supplied only by the second measuring procedure.In order to rule out the use of incorrect absolute values during thecalibration, it can also be provided that the time sequence of theabsolute values be considered during the calibration.

The approach to the obstacle described above does not occur in everycase in which the distance is supposed to be measured. This means that,for example, it may be that an obstacle comes so close while the sensorsare switched off that when they are switched on it is not located in themeasuring range according to the first measuring procedure. Then nocalibration can occur, either. In order to obtain a display that can beevaluated in all cases, it is therefore provided according to anotherfurther development of the present invention that with distances lyingoutside the measuring range of the first measuring procedure, calibratedrelative values are output that were determined according to the secondmeasuring procedure, if absolute values were determined according to thefirst measuring procedure, and that otherwise a warning signal is outputindependently of the relative values determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block wiring diagram of a device for carrying out theprocedure according to the present invention.

FIG. 2 shows an illustration displaying the limits of the measuringrange according to a conventional procedure.

FIG. 3 shows the arrangement of sensors on a motor vehicle for carryingout the procedure according to the present invention.

FIG. 4 shows a representation of absolute values determined according tothe first measuring procedure of the present invention as a function ofthe distance.

FIG. 5 shows several characteristic curves of a capacitive sensor to becalibrated.

FIG. 6 shows an embodiment of a computer program for carrying out theprocedure according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the arrangement shown in FIG. 1, two ultrasound sensors 1, 2 areprovided, each of which receives an alternating voltage from anamplifier 3, 4 for transmitting and each of which is connected to areceiving circuit 5, 6. Ultrasound sensors 1, 2, amplifiers 3, 4 andreceiving circuits 5, 6 are known per se and do not require furtherexplanation in connection with the present invention. The control ofamplifiers 3, 4 and the transmission of the output signals of receivingcircuits 5, 6 occur from and to a control unit 7 with the use of a CANnetwork (CAN=Controller Area Network) which has a CAN control unit 8, 9,10, 11 for each of the connected components. A similar network isdescribed, e.g. in Lawrenz, W., et al.: "CAN-Control unit Area Networkfor In-vehicle Network Applications," SAE Information Report J1583,Botzenhardt, W. et al.: "Bus System for Vehicle Control Modules," VDIReports No. 612, 1986, Pages 459 to 470 and Lawrenz, W.:"Entwicklungswerkzeuge fur Controller-Netzwerke" (Development Tools forControl unit Networks), Elektronik 16/Sep. 4, 1987, Pages 136 to 140. InFIG. 1 only those CAN control units are shown that are associated withthe device used to carry out the procedure according to the presentinvention.

Control unit 7 contains a microcomputer 12 and memory 13. A program formicrocomputer 12 carries out the individual steps of the procedureaccording to the present invention. A display device 14 and an acousticsignaling device 15 are connected to one output of control unit 7.

FIG. 2 shows a diagram of a motor vehicle 21, on the rear end of whichtwo ultrasound sensors 1, 2 are mounted.

Ultrasound sensors 1, 2 have a sensing angle, outside of which obstaclescannot be sensed. These areas 22 are shown shaded in FIG. 2. Inaddition, within the sensing angle in the area of the sensor there arealso other areas 23, 24, in which distance measurement is likewiseimpossible. In FIG. 2, a control unit 16 and a display device 17 arealso indicated.

In motor vehicle 25, shown in FIG. 3, equipped for carrying out theprocedure according to the present invention two ultrasound sensors aremounted on the rear end. In addition, a capacitive sensor 26 extendsover the entire rear bumper.

As shown in FIG. 1, capacitive sensor 26 is formed by an electrode 27that is connected with the input of an amplifier 28 and, using aresistor 29, to a sine-wave generator 30. The output voltage of theamplifier 28 and the voltage of sine-wave generator 30 are supplied to aphase comparator circuit 31. The phase difference depends on thecapacitor formed by electrode 27 to ground. In the ideal case, inaddition to electrode 27, this capacitor is formed also by the areabehind the vehicle with the obstacles to be recorded as acounter-electrode.

However, since body parts, in particular the bumper of the vehicle, aremounted very close to electrode 27 for design reasons, the capacitanceformed by the above arrangement would be significantly higher. Thus inorder to reduce this effect, a shield 32 is provided that is mountedbetween electrode 27 and the conductive vehicle parts and receives avoltage that is preferably substantially equal to the voltage at theelectrode. This is achieved with a gain of v=1 of amplifier 28.

In spite of this measure, the capacitance to be measured with phasecomparator circuit 31 is not only dependent on the distance d ofobstacle 33, but in particular on its size, shape and dielectriccoefficient. At greater distances and/or with smaller obstacles, thecapacitance between electrode 27 and the roadway also has to beconsidered. A signal corresponding to the phase difference is suppliedto CAN control unit 10 via an analog/digital converter 34 and sent on tocontrol unit 7.

In FIGS. 4 and 5, the relationships to distance of the output signals ofan ultrasound sensor and of a capacitive sensor are contrasted in arange from 0 cm to 150 cm. In this process, the output signal A of theultrasound sensor is an absolute value and is shown in FIG. 4 ascalibrated in cm. FIG. 4 shows that a linear relationship exists betweenoutput signal A and distance d. However, depending on the details of theultrasound sensor design, the smallest measurable distance lies between25 cm and 40 cm. Output signal C of the capacitive sensor is shown inFIG. 5 as capacitance change compared to a capacitance that is measuredwith an obstacle located at a very great distance. The function for therelationship to distance d corresponds in the first approximation to ahyperbola, but can deviate from that, depending on shape, position andsize of the obstacle.

FIG. 5 shows measured characteristic curves 41 to 44 for variousobstacles. In this case, it is entirely possible that the characteristiccurves intersect. In the example shown for the characteristic curves ofthe capacitive sensor, the measuring range can be given at about 0 cm to60 cm distance, since there are signals here that can be evaluated anddiffer sufficiently from 0. Thus the measuring ranges of the ultrasoundsensors overlap with that of the capacitive sensor between 25 cm to 40cm and 50 cm. Within this area, calibration of the relative valuesdetermined by the capacitive sensor is possible because of the absolutevalues of the ultrasound sensors. Calibration can be carried out, forexample, by selecting one of the characteristic curves that is valid forthe distances measured with the ultrasound sensors and the values ofoutput signal C that are present at the same time.

If, during an approach to an obstacle, the distance becomes less than 60cm, several measurements of the ultrasound sensors and of the capacitivesensor within the overlapping area will be read and used as inputvariables for the map according to FIG. 5--namely the measurements A ofthe ultrasound sensors as values for the distance and those of thecapacitive sensor as output signal C. Let us assume, for example, thatthis results in points 51, 52, 53, 54, 55. Points 51, 52, 54, 55 lie onthe characteristic curve 42 so these are used for calibration of theoutput signal of the capacitive sensor during further approach to theobstacle. Point 53 lies too far away from the characteristic curve so itis interpreted as an incorrect measurement. In the subsequent usage ofcharacteristic curve 42 for distance measurement with the capacitivesensor, the output signal of the sensor and/or the change in thecapacitance value is compared to the stored table and the distance isread out.

The process steps shown in FIG. 6 as a flow chart for microcomputer 12in control unit 7 will be started after being switched on at 61. In afirst subprocess 62, the measurements of the two ultrasound sensors 1, 2are recorded and an absolute value A for the respective distance iscalculated by triangulation. In addition, the capacitance value C isrecorded. Values A and C, together with a given time t, are stored in amemory.

Drift compensation of values A and C occurs at 63. For the sake ofsimplicity, the compensated values are also indicated in the followingwith A and C. The drift compensation is required, for example, due todependence on temperature.

At 64, a decision is made depending on whether a correlation existsbetween the two values A, C recorded at 62 and the previously recordedvalues--if such are present--and the calculated parameters fromcalibration, e.g. the selection of the characteristic curve 42. If thereis no correlation, an abrupt change compared to the last measurementwill be assumed. An obstacle, which for example, was not present duringthe last program run has appeared when the program is run again. Inthese cases, a warning tone will be output at 65.

A check is carried out at 66 to see whether or not the ultrasoundsensors have sensed an object yet. If they have not, the program isrerun, starting with subprogram 62.

Otherwise, if the ultrasound sensors have sensed an obstacle in previousprogram runs, it branches off again at 67, depending on whether theultrasound sensors continue to sense an obstacle. If this is the case,the distance measured with the ultrasound sensors between vehicle andobstacle will be output at 68. In addition, calibration will be carriedout at 69, e.g. the selection of one of the characteristic curves shownin FIG. 5.

However if it is determined at 67 that the ultrasound sensors no longersense any obstacle, it is a case in which during one of the earlierprogram runs the ultrasound sensors sensed an obstacle and calibrationwas carried out in subprogram 69. This calibration will then be used at70 to calculate distance D determined by the capacitive sensor, whichwill be output. After both subprogram 69 and subprogram 70, a check willbe carried out at 71 on whether the calibrated signal D has reached avalue Dmin. If this is the case, a warning, e.g. the display of the wordStop, will be output at 72. However, if Dmin has not been reached yet,the program will be rerun starting at 62, without any output.

We claim:
 1. A process for measuring a non-contact distance to at leastone obstacle, comprising the steps of:measuring a capacitance between anelectrode and the at least one obstacle; determining absolute values ofthe non-contact distance in a first measuring range using a firstmeasuring procedure; determining relative values of the non-contactdistance in a second measuring range using a second measuring procedure,the second measuring procedure including the measurement of thecapacitance, the second measuring range overlapping with the firstmeasuring range to form an overlapping area; calibrating the relativevalues using the absolute values arranged within the overlapping area;storing, in a map, a relationship between the relative values and anactual distance, the map including a plurality of characteristic linesformed by individual measured points; selecting a characteristic line ofthe plurality of characteristic lines to be calibrated using at leastone of the absolute values; and utilizing the characteristic line in atleast one further measurement using the second measuring procedure. 2.The process according to claim 1, wherein the non-contact distanceincludes a further distance from the at least one obstacle in animmediate vicinity of a motor vehicle.
 3. The process according to claim1,wherein the first measuring procedure corresponds to a delay timemeasurement of emitted, reflected and received waves, and wherein thesecond measuring procedure corresponds to a measurement of at least oneof an electrical variable, a magnetic variable and an optical variable.4. The process according to claim 3, wherein the first measuringprocedure is performed using ultrasound waves.
 5. The process accordingto claim 1, further comprising the steps of:determining calibratedrelative values using the second measuring procedure; outputting thecalibrated relative values for distances outside the first measuringrange if the absolute values have been previously determined using thefirst measuring procedure; and if the absolute values have not beenpreviously determined using the first measuring procedure, generating awarning signal separately from the determined relative values.
 6. Aprocess for measuring a non-contact distance, comprising the stepsof:measuring a capacitance between an electrode and at least oneobstacle; determining absolute values of the non-contact distance in afirst measuring range using a first measuring procedure; determiningrelative values of the non-contact distance in a second measuring rangeusing at least one of the first measuring procedure and a secondmeasuring procedure, at least one of the first measuring procedure andthe second measuring procedure including the measurement of thecapacitance, the second measuring range overlapping with the firstmeasuring range to form an overlapping area; calibrating the relativevalues using the absolute values arranged within the overlapping area;determining a relationship between the relative values and an actualdistance; forming a calibration factor using a predetermined equation toform a plurality of characteristic lines formed by measured points, thecalibration factor being calculated for a calibration procedure byinputting at least one of the relative values determined using the firstprocedure and then utilized using the second measuring procedure;selecting a characteristic line of the plurality of characteristic linesto be calibrated using at least one of the absolute values; andutilizing the characteristic line in further measurements using thesecond measuring procedure.
 7. The process according to claim 4, whereinthe absolute values arranged within the overlapping area are used forthe calibration procedure.
 8. The process according to claim 7, furthercomprising the step of:discarding selected absolute values that areoutside a predetermined threshold range.
 9. The process according toclaim 7, wherein, during the calibration procedure, a time sequence isutilized.
 10. The process according to claim 8, wherein, during thecalibration procedure, a time sequence is utilized.
 11. The processaccording to claim 6, further comprising the steps of:determiningcalibrated relative values using the second measuring procedure;outputting the calibrated relative values for distances outside thefirst measuring range if the absolute values have been previouslydetermined using the first measuring procedure; and if the absolutevalues have not been previously determined using the first measuringprocedure, outputting a warning signal separately from the determinedrelative values.